(1) Field of the Invention
This invention relates to a thin transparent polytetrafluoroethylene film having high transparency and strength and a production process thereof.
(2) Description of the Prior Art
Polytetrafluoroethylene (hereinafter called "PTFE") is widely used owing to its various merits, including its large electrical resistance, low surface tension and excellent mechanical properties in addition to its high heat and chemical resistance. It is however difficult to form PTFE into a thin film by the melt-forming technique due to its high melting point (327.degree. C.) and high polymerization degree. As also well known, it is difficult to form PTFE into a thin film by the casting process because due to lack of suitable solvent, it is also difficult to dissolve PTFE into a solution. Although it is possible to obtain thin films having thicknesses of about 25 .mu.m by cutting work or the like, the resultant thin films are usually white and opaque since such a method is susceptible of developing microvoids and PTFE is highly crystalline.
It has recently been proposed, as a process for obtaining formed PTFE articles, to fluorinate formed articles of a fluoroethylene polymer with fluorine gas so that the polymer precursor is converted to PTFE while kept in the shape of its formed articles [see for example Shinozaki, Iwasaki, Okazaki, "Preprint of the 29th Autumn Annual Conference of the Japan Chemical Society", 3H13 (October, 1973)]. There are available a variety of soluble or meltable polymer precursors. Therefore, thin PTFE films can be obtained when such soluble or meltable polymers are selected and their thin films (for example, thin cast films formed from their solutions) are then fluorinated. However, as far as the present inventors are aware of, thin PTFE films obtained in accordance with the above process are generally insufficient in mechanical strength and are so fragile that they are readily broken even when held by hands.